Variable pitch propeller blades are commonly used on propeller-driven aircraft. They allow the propellers to adapt to different thrust levels and air speeds, thereby improving the propulsion system's efficiency.
Variable pitch fan blades have also been proposed for turbofan engines. For example, the Rolls-Royce M45SD-02 engine described in Flight International, 19 Apr. 1973, pages 615-618 had a variable pitch fan. Advantages of such engines include: reduced noise levels because of ability to lower fan tip speeds, rapid thrust response, possibility of producing thrust reversal without the weight and maintenance penalty of more conventional thrust reversers, high ratio of take-off to cruise thrust, good cycle efficiency and specific fuel consumption, and wide engine operating margins.
However, a problem with implementing variable pitch blades is that there must be adequate safety features to mitigate against possible failures. This is particularly the case with variable pitch systems incorporating thrust reversal. For example, the overlapping blade nature of modern low pressure fans means it is not physically possible to lower the blade pitch through zero pitch to a negative angle. In any event, a transition from a positive to a negative pitch angle (even if possible) would carry a risk of fan overspeed. Accordingly, the only realistic option with such engines is to increase pitch angle through the feather angle at about 90°, to achieve reverse thrust at around 150°. However, without adequate safety features if the primary actuation fails, the blade angle will either decrease resulting in rotor overspeed, or increase to give reverse thrust, depending on which side of zero resultant fan torque the fan is at the point of failure. Failure to the reverse thrust position will result in the end of a flight. Failure to fine pitch will result in fan overspeed, with possible consequences of blade release and loss of the engine function.
It is thus desirable to provide a reliable actuation system for varying blade pitch which is tolerant of faults in the system.